Adding effects in background?

I am trying to add a effect to a movie that I have imported into motion. The effect is an explosion, but I want to have part of it happen behind an object.

I've seen this done in Adobe AE singling out the object/objects using Track Mattes. Can someone please guide me in the right direction on what approach I should be taking to do this. I'm sort of a noob in motion.

Nothing green screen, just a regular video clip, which is what I'm confused about. Do I have to overlay/underlay anything for when I use the mask tool? Everything I have tried so far I have been unsuccessful, but I don't know if that's because I need to have a layer underneath the mask.

Ok since your footage is not keyable. You need to duplicate it. So you have 2 layers of the exact same thing. Then on the top layer you want to roto/mask out the background area that you don't want. Essentially you are seperating the element you want something to happen behind by making it a seperate layer. (tip even though you've duplicates your layer turn off the bottom layer for bow so you can see how the mask works.) now put you composited element between the 2 background layers Now you have your effect behind a person or object. To have it it on top you add another layer.

Basically you should have a project that has a layer structure like this:

effect (on top of everything)
footage (part of it masked)
effect (showing though the masked area)
footage (so you still have a background)

hopefully that helps.

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Posted from my iPhone, so please for give any typos or run on sentences.